“O Lord, save the king! May he answer us when we call.” -Psalm 20:9
This closing petition is an inclusio to verse one: “May the Lord answer you in the day of trouble! May the name of the God of Jacob protect you!” -Psalm 20:1
The last verse echoes the first, but expands on it’s first meaning. The people, in praying for the king’s salvation in the day of battle, secure their own salvation. King David, being the official representative of God’s people—Cuius regio, eius religio—seeks God’s favor and protection on behalf of the nation; upon realizing the answer to his prayer, the king, being saved in battle, is strengthened and equipped to hear the people’s prayer to him and answer them.
Additionally, Calvin’s thoughtful exposition and Christological application of the passage bears repeating here:
“In the present day, when Christ is now manifested to us, let us learn to yield him this honour—to renounce all hope of salvation from any other quarter, and to trust to that salvation only which he shall bring to us from God his Father. And of this we shall then only become partakers when, being all gathered together into one body, under the same Head, we shall have mutual care one of another, and when none of us will have his attention so engrossed with his own advantage and individual interest, as to be indifferent to the welfare and happiness of others.”